In certain types of air interfaces, parameters that define how communications are conducted on the downlink and/or uplink can be adjusted dynamically to adapt to changing radio frequency (RF) conditions. Such dynamic adjustment of parameters can involve adaptive coding and modulation (ACM) techniques, in which the type of modulation and/or coding is adjusted based on RF conditions. Changing the modulation and/or coding that is used over a communication link (such as the uplink or downlink) can result in a change in the effective data rate over the link. For example, if RF conditions are poor, the modulation and/or coding may be adjusted to make the wireless signals less sensitive to noise, fading, or other adverse RF effects, but at the expense of a lower effective data rate. On the other hand, if RF conditions are good, the modulation and/or coding may be adjusted to allow for a higher effective data rate, but at the expense of having the wireless signals more sensitive to noise, fading, or other adverse RF effects.
To adapt downlink communications to RF conditions, a wireless communication device (WCD) can transmit to the radio access network (RAN) an indication of the modulation-and-coding scheme (MCS) to be used for the RAN's downlink communications to the WCD. This approach is used in, for example, Long Term Evolution (LTE) air interfaces. LTE defines a series of channel quality indicator (CQI) indices for controlling the MCS and, hence, the data rate used for downlink communications. The CQI indices for downlink communications range from 0 to 15 (a CQI index of 0 indicates no transmission), with higher data rate indicators being associated with MCSs that support higher downlink data rates. In operation, a WCD may measure a signal-to-noise ratio (SNR) of a reference signal transmitted by the RAN, select a CQI index based on the measured SNR, and report the selected CQI index to the RAN. In response, the RAN may transmit downlink data to the WCD using the MCS associated with the CQI index that the WCD reported.